1. Field of the Invention
The present invention relates to a technology of creating a print image by creating a copy prevention pattern, a latent image, which comes up at a time of copying is embedded, into a mask pattern, into which a special dot pattern for preventing copying is incorporated, and superimposing the copy prevention pattern onto a document image.
2. Description of the Related Art
As a method for preventing copying of a confidential document and the like, there has been known a method of using a pattern image which causes warning characters to appear if copying of the confidential document is performed in a copying machine (Japanese Patent Unexamined Publication No. 2005-235181). Creating a document with use of such pattern image has an effect of psychologically preventing unfair copying and distinguishing an original document from a copy.
However, even though the method using the pattern image has an effect of psychologically preventing the unfair copying, it cannot physically prevent the copying, and leaking of information cannot be prevented completely. Therefore, there has been a known copying machine having a copy guard function for preventing leaking of information of a document. According to the copy guard function, a mask pattern, into which a special dot pattern is incorporated, is printed together with a document image, and when the document is to be copied, the copying machine identifies the dot pattern to prohibit copying of the document or forms a solid image without printing the document image. This copy guard function is realized only with a copying machine which stores the special dot pattern and has a function of detecting the special dot pattern from image data obtained by reading of the document. In other words, if copying of a document, on which a mask pattern having the special dot pattern is printed, is performed with a copying machine which does not have the copy guard function, the mask pattern is read as a mere image, and copying can be performed as usual.
Therefore, as shown in FIG. 7, there is a case where a method combining the mask pattern and the latent image is used. In particular, a document 91, in which a latent image is embedded into the mask pattern, is created. When the copying machine 92 does not have the copy guard function, and the document 91 is copied, a document image is printed so that characters of “copy prohibited” come up together like the copy 93. On the other hand, when the copying machine 92 has the copy guard function, printing the document image is not performed, and alternatively, for example, a solid image like the copy 94 is printed.
The mask pattern into which the latent image is embedded will be described as “copy prevention pattern” hereinafter. In the copy prevention pattern before being copied, difference in densities of the mask pattern and the latent image is so small that the copy prevention pattern is identified as normal pattern when viewed with naked eyes.
In the copying machine having the copy guard function, the special dot pattern is detected from image data which is obtained by scanning the document. Then, the copying machine reads a predetermined number of the special dot patterns, and the pattern printed on the document is determined as a copy prevention pattern, and copying of the document is prohibited. However, if another image such as the latent image is printed on the mask pattern, there may be cases where detection of the special dot pattern is interfered so that the time for detection becomes long, and detection of the special dot pattern is failed, so that copying may be preformed mistakenly. The above-described Japanese Patent Unexamined Publication No. 2005-235181 discloses the technology of designating print area of a pattern image including the latent image. However, it is necessary that area designation is performed by the user, so that it was time-consuming.